1. Field of the Invention
The invention relates to the field of environmental metrology. It concerns, in particular, a process for investigating particles situated in a gas, in which process the gas containing the particles is guided in the form of a gas stream past a UV light source, the particles are ionized by the UV radiation from the UV light source, the ionized particles are filtered out from the gas stream and the photoelectrically induced current which is generated is measured.
Such a process is disclosed, for example, in U.S. Pat. No. 4,837,440 or DE-A-41 13 929.
2. Discussion of Background
On account of the increasing burdening of the environment with pollutants which, for example, are emitted by oil-fired heating systems or car and commercial vehicle traffic into the air, simple and reliable methods and equipment for measuring and monitoring the air quality are becoming progressively more important.
In this case, particular significance is ascribed to the particles which are in suspension in the air and which can be utilized for measuring the air quality for the following reason: in the course of any combustion of fossil fuels, very small carbon particles (.phi.&lt;0.1 .mu.m) are generated. In the course of the cooling of the combustion gas, a condensate may form on the surface of these particles, which condensate includes, inter alia, so-called polyaromatic hydrocarbons (PAHs). The concentration of these PAHs is a good measure of the quality of the combustion; it also shows good correlation with other pollutants which are formed in the course of a poor combustion, especially with CO. By measuring the PAH concentration, it is thus also possible to make a statement concerning the quality of the air.
A known and acknowledged method for measuring the PAH concentration is based on the following principle: by irradiation with UV light (wavelength for example 185 nm), it is readily possible to ionize particles on the surface of which PAHs are situated. If the gas which contains such ionized particles is conducted, after separating off the electrons formed in the course of the ionization, through particle filter carrying (ion) filter which is mounted in an electrically insulated manner and is connected to a current amplifier, the current caused by the charged particles can be measured. This current is a measure of the concentration of particle-bound PAHs.
In the known measurement processes, low-pressure mercury vapor lamps are employed in the charging unit. The emission of these Hg lamps is principally in the region of 253.7 nm and is thus of too long a wavelength, in the case of many aerosols, to initiate photoemission. That part of the spectrum which can be used for these purposes is around 184.9 nm and is, in its intensity, more than 10 times smaller than the longer-wavelength part. Over and above this, UV radiation of 184.9 nm generates ozone. The measurement involves greater difficulties, since the inapplicable part of the spectrum must be filtered out. The UV intensity is, moreover, greatly dependent upon the temperature and the number of operating hours of the respective lamp. A UV power value which is to some extent stable is not obtained until approximately 30 to 60 minutes after switching on.
As the radiators employed hitherto are available only as external radiators, the charging efficiency is also relatively low.